One of the biggest problems in mobile telephone systems is an efficient use of a limited radio resource. In present systems, a certain quantity of resources, for instance one-spreading code, is reserved for each user for a circuit switched call during the entire radio link. The problem occurs, because not only is user data continuously transmitted, but system control data must also be transmitted at sporadically or regularly repeating intervals. Another problem relates to the operation of user equipment in slotted mode, in which the user equipment measures the received power of other frequencies of adjacent base transceiver stations for part of the duration of the radio frame. In this case, the user equipment cannot receive normally, but may even loose the entire radio frame.
For instance, in systems using the code division multiple access method (CDMA) one spreading code with spreading factor 256 is reserved for each connection on a downlink from the radio network subsystem to the user equipment. This spreading factor is enough to transmit user data, but when it is necessary to transmit control data, coding must be cut down too much. One solution to the problem is to replace user data with system control data, but in this case some of the user data would be lost, and when transmitting speech, for instance, this would be noticeable as a decrease in speech quality, even breaking.
Another solution to the first problem is to reserve data transmission resources in a manner that there is enough of them all the time. In this case, a spreading code with spreading factor 128, for instance, is reserved for the connection. A problem occurs, however, because the need for transmission resources is calculated on the basis of the momentary maximum required and thus transmission resources are wasted during most of the time when control data need not be transmitted. In theory, it is possible to use 128 different spreading codes simultaneously, but, as in certain cases even 25 to 30% of users are performing soft handovers at the same time, there are, in practice, less than a hundred spreading codes left to use.
Used spreading codes can be arranged in a code tree in which the spreading codes used in the system are arranged mutually orthogonally. When moving deeper into the tree, the length of the spreading codes doubles on each level, thus halving the data transmission speed. One suggested solution to the second problem above is that adjacent spreading codes on one level, i.e. sibling codes, be reserved for two different users. The spreading factor of the sibling codes can be 256, for instance. When necessary, one of the users can then obtain the parent code of said spreading codes, i.e. a code one level up with spreading factor 128. However, this solution entails that the users are synchronized with each other, because the first user cannot use its own sibling code while the second user uses the parent code. The first user equipment can thus make measurements during the first half of a normal frame and receive a shortened frame spread with the parent code during the second half, in which case its data transmission capacity corresponds to a normal frame spread with the sibling code. The second user equipment receives a shortened frame spread with the parent code during the first half of the frame and makes measurements during the second half. In normal operation, both sets of user equipment receive normal frames spread with their own sibling codes. The required synchronization is a considerable limitation to the flexibility of the system.